1. Field of the Invention
The present invention relates to a data conversion method for displaying an image with gradation by controlling a light emission time per one frame and a display device that uses the method. The invention is suitable for a plasma display panel (PDP).
A PDP has both a high speed property and a high resolution necessary for a large screen display device of a TV set or a monitor display of a computer. One of the tasks of developing such a PDP is to reduce pseudo contours in displaying a moving image.
2. Description of the Prior Art
A half tone is reproduced in a PDP by setting the number of discharges of each cell (each display element) for one frame in accordance with a gradation level. A color display is one type of the gradation display, and a display color is determined by combination of luminance values of the three primary colors.
A gradation display method for a PDP is known, in which one frame is made of plural subframes having weights of luminance, and the total number of discharges of one frame is set by combining lighting and non-lighting of each subframe (referred to as a subframe expression). In general, conversion of a frame into subframes is performed by using a conversion table that is prepared in advance. Furthermore, in the case of an interlace display, each field of a frame includes plural subfields, and each subfield is controlled for lighting. However, the lighting control is the same as that of a progressive display.
In a display using a light control of subframe unit, lighted subframes and non-lighted subframes are mixed so that light emissions occur at discrete timings in the frame period. Thus, a pseudo contour can be generated. A pseudo contour is a phenomenon in which an observer sees light and shade different from the display contents, and can be generated easily when a portion of an image having pixels of similar gradation levels constituting a gentle gradation change moves in a screen. For example, in a scene with a walking human body, a pseudo contour can occur in a face of the human.
Conventionally, a method of reducing pseudo contours is known in which the weighting is devised so that plural subframe expressions are possible for a half tone, and an optimum subframe expression is selected for each gradation level by noting each frame. A basic rule of optimizing the subframe expression is to stabilize the light emission barycenter in a frame period regardless of the gradation level as disclosed in Japanese unexamined patent publication No. 10-307561. For example, the light emission barycenter is set to be always in the middle of the frame period. If the light emission barycenter is constant, an interval of the light emission barycenter between frames becomes constant, so that a deviation of the light emission timing such as a long period of low luminance can be eliminated.
Moreover, Japanese unexamined patent publication No. 11-224074 discloses a method of selecting an optimum subframe expression, in which a frame to be converted into subframes (referred to as a current frame) is given a subframe expression by referring to a subframe expression of the previous frame and considering the relationship between the previous frame and the current frame. This method can reduce pseudo contours more securely than the method of determining the subframe expression by noting only the current frame.
Conventionally, it is necessary that a skilled person decides a subframe expression to be selected for each gradation level based on the person's experience when making a conversion table for coordinating a frame and subframes in order to reduce pseudo contours substantially. Especially, if the relationship between the previous frame and the current frame is considered as mentioned above, an optimum subframe expression should be determined for each of 2562 combinations of gradation when the number of gradation N equals to 256, so a vast labor is necessary. In addition, if two or more previous frames should be referred to, the number of combinations of gradation is up to N3. If a specification is revised by increasing the number of gradation N or changing the weighting, the bothersome job is necessary.